Wow, thank you, i think i understand and i also totally agree the marketing strategies of some tech manufacturers (for example i dont like how canon adds so many mega pixels on some cameras that are not sharp at all). When i started photography i knew there were a lot of things to learn but day by day i am amazed of the amount of things that exist and has to be learned and i never realized they exist even if they are in front of me (literally) all the time . Optics is one of these things.
I might be wrong but here is what i have understood, in a physical point of view, the lens has, lets say, 1 billion glass atoms, a lens with more atoms is sharper than a lens with less atoms. A FX sensor at will use all the atoms but a DX sensor will use (because its crop is 1.5 times smaller than a FX) only 666666666.(6) atoms so it will be less sharper because less atoms means less lens resolution at any F. If i am right than i wish the camera had a option to be able to move the sensor backwards to be able to use the full sharpness of the lens when you use a full frame lens on a DX .
But i have a few more question, why does this happens especially on wide lenses or why on this wide lens .
Speaking in general there are high quality ultra-sharp lenses like 14-24mm, 28-70mm f/2.8 and some others which can fully use potential of high mp-count full frame sensors like installed in D800. Other, more versatile lenses such as 24-120mm, 24-85 may be good on full frame but rather low megapixel D700 12mp, but will be limiting factor on D800 with 36mp. Every lens has it's limit on resolution which correlates with sensor limits. In your case, the limiting factor is DX sensor in D7000 which is not able to full realize potential of this lens. And to be sure that the lens itself is not limiting compare
for example D7000, D700 and D4 (D800 would be better but it's not tested with this lens) and you will see that D4 with this lens has more that twice as much score (24 vs 11), but score is not everything. If you look on resolution data, you can see that it is higher on camera with more mpx within sensor size of course. For example D7000 - 16mpx DX sensor gives only 44lp/mm, D700 - 12mpx - 46lp/mm, new D4 - 16mpx - 52lp/mm, and rather old but with higher quantity 24 mpx - D3X - 57lp/mm. So i guess the absolute winner in terms of resolution will be D800 - 36mpx sensor, but it's rather interesting to see, how much better can this lens perform on D800 comparing to D3X or D600.
So if we take into account that this lens was developed specifically to be used with full frame sensors, it's not surprising that you have less picture quality on DX camera though it has more mpx than D700 you were comparing with.
P.S:If to speak about DX i will not be surprised if this lens will perform better on Nikon D3200 24mpx sensor)
P.S 2:But sometimes it's difficult to understand DXomark rating. For example if we take Samyang 14mm f/2.8
and compare D700 vs D7000, we will see that all
Lens Metric Scores are better for D7000 but still D700 has higher score with this lens.
So there are cases when DX sensor obtains better resolution and better picture quality than FF.